The present invention relates to a method of controlling the speed of rotation of a turbine, in particular a wind turbine, in such a way that a higher energy production at a wide range of wind speeds, in particular at low wind speeds, is obtained, as well as an improved quality of the produced electricity, and at the same time reducing the structural loads on the turbine. Further, a turbine being operated according to the method according to the present invention is easier to control than existing turbines. A method according to the invention is particularly useful when operating wind turbines, such as stall controlled wind turbines.
The present invention further relates to a turbine, in particular a wind turbine, the turbine being capable of being operated according to the method according to the invention.
The present invention even further relates to a method of operating a plurality of turbines, in particular wind turbines, e.g. forming a wind farm, such as an offshore wind farm.
It is known to control wind turbines according to a stall control strategy. However, it is desirable to provide a control strategy in which an even higher energy production may be obtained while still maintaining control of the turbine and ensuring that the turbine is not damaged during the normal operation. It is also desirable to provide a control strategy in which the parameters used for controlling the operation of the turbine are readily and easily measured and/or estimated. In normal stall controlled turbines the small power fluctuations are provided. However, this causes the structural loads on the turbine to be large. It is therefore desirable to provide a control strategy in which the structural loads on the turbine are reduced while it is ensured that the power fluctuations remain small.
In “Autonomous wind energy conversion system with a simple controller for maximum-power transfer”, IEE Proceedings-B, Vol. 139, No. 5, pages 421-428, Stevenage, Sep. 1992, by M. Ermis et al., there is disclosed a method of controlling a wind turbine. The method optimises the energy output of the turbine for a chosen wind regime while keeping the generator cost at a minimum. This is obtained by choosing the power output as the controlled variable. Thus, a rotational speed is measured and ‘processed’ in order to obtain a corresponding power value. This step is performed by means of a look-up table.
In “Control Strategy for Variable-Speed, Stall-Regulated Wind Turbines”, Proceedings of the 1998 American Control Conference, Vol. 3, 1998, pages 1710-1714, Philadelphia, USA, by E. Muljadi et al., there is disclosed a method of operating a wind turbine in which a torque is measured and processed in order to obtain a speed of rotation. At low wind speeds the wind turbine is operated so that maximum power is produced by operating near Cp,max. At higher wind speeds the wind turbine is prevented from following the Cp,max trajectory and is forced to operate at a lower tip-speed-ratio and Cp. It is thus not possible to obtain maximum power output at higher wind speeds.
In GB 2 206 930 A there is disclosed a wind turbine operating system comprising a speed limiting system. When wind speed increase tends to run the turbine above its rated conditions, the brake is applied to slow the turbine to a speed at which the blades are operating so inefficiently that further wind speed increase cannot increase shaft speed or torque unacceptably.
None of the above references discloses a method for controlling a turbine in which an optimal energy production may be obtained at higher wind speeds.